There isn't anything specific about loans but this is what has been in the roster rules and regulations page for a couple of season:
https://www.mlssoccer.com/about/roster-rules-and-regulations
Related Party Transactions
All terms of any arrangements among an MLS club, a related party club, and a player to be signed to MLS shall be fully disclosed to the League. A player joining an MLS club from a related party club will calculate his Salary Budget Charge based on the compensation he is receiving from his related party club contract and any acquisition fees associated with his signing. The League will make a final determination, in its sole discretion, as to the Salary Budget Charge of such a player.