Are letting agents allowed to keep my tenancy deposit after I’ve moved out in lieu of another person’s payment?

I moved out of my house 2 months ago which was on a joint tenancy agreement. The letting agent’s hold £450.00 deposit per tenant, but they hold £900.00 in my name because I was covering someone who wasn‘t going to be on the contract, who has also moved out. The deposit is on a deposit protection scheme. They have not informed me of any deductions to the deposit. However they have said that they will pay me £450.00 now and the remainder when the deposit from an incoming tenant is received. Are they allowed to keep my deposit in lieu of another person’s payment?