The Mishna (83b) teaches that in a case where a person takes a neder that kohanim and levi’im cannot derive any benefit from him, the priests and Levites can take the teruma and ma’aser that are due to them, even against the will of the one who made the vow; the tithes, after all, do not really belong to him – they belong to the families of the priests and Levites. If, however, the person says that specific kohanim or levi’im cannot derive benefit from him, then those priests and Levites should not take tithes from him, but rather should allow others to do so.
The concept of tovat hana’ah is the benefit of discretion, and it would seem from our case that it has a real monetary value (not a mere sense of pleasure for having done something) that can be associated with it. This can be evaluated by answering the question – is this something that a person can get paid to do? Specifically in our case, the owner of the field who is dividing up his tithes among kohanim and levi’im of his choosing can be approached by someone who is willing to pay him to give the tithes to specific people. Do we view this as an intrinsic part of the value of the item, or is it, at best, a side benefit that has no specific significance?
The Talmud Yerushalmi brings a disagreement as to whether a person is allowed to accept tovat hana’ah payment in the case of tithes. All are in agreement, however, that if such a payment can be made, it must be made by a third party and not by the recipient himself, since such an arrangement would be hillul kodashim – a desecration of the tithes.