An exchange agreement is known in legal Spanish as a “contrato de permuta”. When, as in your text, there is an exchange of goods with one of the parties paying an extra monetary amount, this is known as a “permuta con sobreprecio.” The technical term for that extra “purchase exchange price” is “sobreprecio”. Here are a series of references concerning “permutas con sobreprecio” that might provide extra terminology:

https://vlex.es/tags/permutas-con-sobreprecio-2004751