Grotius on 'unevenness'

One fascinating area in Grotius concerns his theory of contract.  I covered some of this material in my Roman law lecture course last spring term.  I used much of the material in the De Jure Belli ac Pacis II.  But now I realize I should probably have integrated more of the material from his outline of Roman-Dutch law, which in general I believe is far clearer in presentation than some of the well known passages in the former.  

One feature that makes this work so valuable to me is the tabular presentation of legal concepts:

Grotius, Hollandsche Rechtsgeleerdheyd

Grotius' reasoning goes as follows:  The two main divisions of the law can be divided into jus reale (corresponding to property law covering matters of legal rights and duties in rem) and jus personale (corresponding roughly to the law of obligations covering matters of legal rights and duties arising in personam).  

His treatment of the latter - legal obligations in personam - is unusual and does not follow the classical divisions of Roman law.  Rather, the main division, as he sees it, is between toe-segging (promissory obligations) and onevenheit (obligations arising from 'unevenness' or 'inequality').  The latter is most interesting - it is where he places the so-called 'innominate contracts' and 'quasi-contracts' as well as most of the categories of delictal obligations in classical Roman law.  The contractual exchanges that Roman lawyers simply called do ut des ('I give something, just as you give me something'), facio ut facias ('I do something, just as you do something for me'), do ut facias ('I give you something, just as you do something for me') all fall under these categories because of the real possibility that such arrangements can easily break down and leave someone hanging.  

Unevenness is a difficult concept to translate.  Grotius sometimes presents it as a matter of 'inequality' in commercial exchanges, when say the value of things exchanged (or services rendered) in a commercial exchange don't quite match up.  It is an old idea whose origin can be traced to Aristotle and Roman law.  The party that receives less in such an exchange has somehow been injured and, thus, entitled, at least in principle, to some sort of recompense, in order to restore equality or 'evenness' in the exchange.  

But the problem of unevenness needn't be restricted simply to commercial exchanges.  Mistakes leading to unjust enrichments and delictal (or tortious) actions creating unevenness can also give rise to obligations that must be performed legally.  These include the usual cases of theft and bodily injury - but even cases of outrages and defamation, injuries on the honor and esteem of others.  

What I find most striking is that, even here, Grotius clings to the idea that, where there is a legal obligation, there must be some corresponding legal right.  Thus, even in cases of 'unevenness,' it is a potential source of a right that restore equality in an exchange - not just commercial, but even social exchanges.  Grotius hints at this in De Jure Belli ac Pacis II, 12.  But this presentation fills in a lot of the blanks.