Plumber not keeping his word

Timothy repaired blocked drains at a complex where I rent a flat (Jan 2021). The work was completed correctly and efficiently. He requested a partial payment, in cash "to help with cash flow". I agreed to assist him by complying. After he had been paid in full by the management company, he did not refund me. This despite repeated requests from me and acknowledgement of debt, apologies and promises on his part.