§1. A person who intends to entrust to others the celebration of Masses to be applied is to entrust their celebration as soon as possible to priests acceptable to him, provided that he is certain that they are above suspicion. He must transfer the entire offering received unless it is certain that the excess over the sum fixed in the diocese was given for him personally. He is also obliged to see to the celebration of the Masses until he learns that the obligation has been accepted and the offering received.
§2. The time within which Masses must be celebrated begins on the day the priest who is to celebrate them received them unless it is otherwise evident.
§3. Those who entrust to others Masses to be celebrated are to record in a book without delay both the Masses which they received and those which they transferred to others, as well as their offerings.
§4. Every priest must note accurately the Masses which he accepted to celebrate and those which he has satisfied.
Source: Code of Canon Law (Vatican.va)