One of the basic differences between the religious and civil laws of the Torah is while the former are mandatory the latter are not necessarily so. Couples who marry may not jointly agree to ignore the laws of kashrut, whereas two people can agree to “make a [monetary] condition against the laws of the Torah.” While the Torah requires a worker be paid daily an employee can agree to being paid monthly. Or, to cite another example, the Torah may require a bailee to take an oath to give weight to his claim that he was not negligent in his duties, but by joint pre-agreement that requirement may be waived. Unlike our bodies (and soul), which are on loan to us by the Creator, the money we earn is considered our own allowing us much greater leeway in our use of it[1],[2]
When it comes to rabbinic civic legislation one can choose to ignore the enactment if such enactment offers no benefit. Just as a football team may waive an offside penalty against the defense if they score a touchdown on the play, one can waive an enactment when one is better off without it. 
The Gemara (Bava Kamma 8b) cites the example of a wife waiving her right of support from her husband. In the Biblical worldview it is the husband’s obligation to ensure the financial well-being of the family. This obligation was independent of the wife being employed outside of the home, a decision left to her to make. As a result of this support the rabbis ordained that all monies earned by the wife would go to the husband - who would presumably use it for the family’s benefit. However our Sages gave the wife the right to opt out of this decree allowing her to keep her own earnings provided she does not ask her husband for support, something akin to separate bank accounts.  
Without this ability to opt out of a rabbinic enactment one could be forced to sell the best of his land to satisfy a debt incurred by the one who sold the land to the unfortunate buyer. Let me explain. 
While a million dollars is a million dollars our Sages understood that there is difference between a million dollar property on Fifth Avenue and one in rural Wyoming (nothing against Wyoming but...) or shall we say between an apartment in the heart of Jerusalem and one in chutz la’aretz even if they may sell for the exact same amount of money. Property was thus divided into three qualitative categories; idit, beinoneet and ziboreet, which can loosely be translated as great, mediocre and not of the best quality. This is a classic case of quality being worth more than quantity. 
In order to encourage one to be careful with the property of others, one who caused damage had to pay “from the best of his fields and the best of his vineyards.” (Shemot 22: 4) On the other hand, Biblically, a creditor was able to collect only from the lowest quality field. However in practice our Sages ordained that they be allowed to collect from beinoneet, fields of mediocre quality. This was done in order to ensure that credit be readily available for borrowers. Lenders fearing that in cases of default they may be able to recover their debts only from with lower level fields would think twice before extending loans and might even refuse to do so[3]
The lower quality fields were to be used to satisfy the debt owing on a ketuba of one’s wife. The reason, the Talmud explains, is that “more than a man wants to marry a woman wants to be married.” (Gittin 49b) A woman is so excited to marry that she is happy to agree at the time of marriage to a ketuba payment of lower quality fields. While this reasoning may not sit well with many moderns it certainly was true in Talmudic times and well beyond. 
In the case where a debtor has only one class of property all debts would have to be settled from that one class even if the land he owns is of inferior quality and he must pay for damages or an outstanding creditor[4]. The creditor has no right to collect from the higher quality land that the debtor owned at the time of the loan but subsequently sold. Jewish law allows a creditor to satisfy his debt from property sold to a third party only if the debtor has no current assets. It would not be fair to collect from a third party when the debtor himself can make payment from his own property.
When the debtor has no assets to satisfy his debts and the creditor collects from those who purchased land subsequent to the debt he must first collect from the last piece of property sold. If there is not enough to satisfy the debt one then moves to the previous purchaser. This creates a problem if the debtor must pay damages, satisfy a loan and pay a ketuba and the debtor has sold his various fields to one person with the best quality being sold last[5].
What should happen is that all three creditors should collect from the last piece of land sold, which in this case is the best land. However such a scenario means that the buyer would be forced to give up his best land despite the fact that he has poorer quality land, which he could give to the creditors and ketuba. Thus we allow the buyer of the land to say to the creditors that they should collect from the land they would normally be entitled to i.e. the debt from the middle land and a ketuba from the inferior land. While this violates the rabbinic enactment of collecting first from the latest property sold, the owner can say the enactment was done for the benefit of earlier purchasers and in this case it works to his detriment as it forces the purchaser to give up the best of his land. Rabbinic enactments were made to benefit people and when they do not it’s best to acknowledge such and apply them in other cases.
[1] We should note that this is actually a matter of Tannatic dispute with the Tanna Kamma asserting that even in monetary law one has no right to waive his rights and no agreement can be made against the monetary laws of the Torah.  
[2] Interestingly one exception to this rule is regarding the charging of interest, which a lender may not charge even if the borrower agrees. While the charging of interest would seem to fall under monetary law, the Torah’s prohibition is rooted in ritual law. One should not charge a “family member” i.e. other Jews interest on a loan. As a privilege of membership in the Jewish people, and being that there is no moral wrong with the charging of interest, one may charge (and be charged) interest to a non-Jew.
[3] We will come across this notion again at the beginning of masechet Sanhedrin where the Sages did away with the need to cross-examine witnesses in monetary cases. They feared that if a dispute arose the creditor would have a difficult time collecting, as the testimony of witnesses might not stand up to cross-examination perhaps due to a technicality. To avoid such, creditors would refuse to lend money “closing the doors for borrowers.” 
[4] Conversely if all the debtor has is the best grade quality land he would have to use that land to satisfy the ketuba of his wife. 
[5] In the ancient world land was generally sold only to satisfy one’s debts and was a sign of near poverty. Hence one would begin by selling the most inferior land and only as a last resort would the best land be sold. We can understand why the Torah instituted that land goes back to its original owner in the Yovel year, allowing one a fresh start, predating the current law of bankruptcy.