"I only use the Rental Property tab functions."
That's is a user choice: it is not a Quicken requirement.
ALL the transactions you enter via the Rent Center are placed into regular Quicken account registers.
You could enter all the transactions you have been entering from the Rent Center, directly into the appropriate account register, if you wished.
But for the purpose of offsetting the purchases you previously entered via the Rent Center, just go to the Quicken account register where those transactions were entered.
If the original transactions were entered (by Quicken when you entered them in the Rent Center) in the Spend column in a Quicken cash account; enter the returns (directly) in that same cash account ... in the Receive column.
Preferably, you would use the same payee, category and tag as you did in the purchase transaction. If you don't know the payee or category, you should at least attempt to use the appropriate Property Tag, if you used one on the purchase.