I'm trying to keep my ex from selling the house I live in until property values go back up. In the divorce decree it states that before any profit is split, my mom is to be repaid $60k for a personal loan. She has now placed a lien on the property.

My question is, in order for him to sell the house for less than the total amount owed, does she have to give permission to accepting a lower price?

Based on the information provided, no.

Separate from the divorce decree it would appear that there is some paperwork related to the loan. What it actually says, may change things. But, all else being equal, if she is owed a personal loan of $60K, has placed a valid lien on the property, and the property is sold so she nets, say only $40K, she can't stop the sale. But, she would still be owed the balance of $20K. It would be different if she held an actual mortgage on the property.

So, this looks like the home is worth nothing to him. Whatever equity is in the house is going to be absorbed by the personal loan or you wouldn't have asked the question. And, I might guess that he is paying expenses that he'd like to get out from under. He, at least, has the loan on the property which would affect his ability to get other loans.

That leaves the question of actual ownership of the property. If you are listed as joint owner, you could stop the sale subject to whatever limitations may be in the divorce decree. If he is sole owner and he isn't limited in the divorce decree other than your mother's loan being paid before you and he split any profits, it seems there is little you could do unless you have other issues in your favor. For example, if there are any terms of the divorce that he is unhappy with that you don't really care about (or care less about than the house), you might suggest that you give in on some totally separate issue in return for him not selling at this time.