It’s definitely up to you! If you want to be modest, you can keep it anonymous. Though Pam’s response depends on whether or not you tell her you bought the house for her.  If you kept it anonymous, Pam would say that whoever built her the house must be an absolute angel, to which Robin jokingly disagrees. On the other hand, Pam will feel quite demeaned and indebted if you choose to tell her you were the one who bought the house. She even thinks of a way she could repay you for your good deed.