Before my father passed away he sold an entire screen printing set-up to a close friend of his for R50 000.

Question

Before my father passed away he sold an entire screen printing set-up to a close friend of his for R50 000. His friend used the equipment for +-9 months running his business, In that time his friend paid him +- R25 000. After 9 months of use his friend returned this machinery as he was closing down this business. The machinery was then put into storage since then. After my father passed away this friend contacted me and demanded R18 000 back from the R25,000 paid to my father, after making full use of the machinery and profiting from it for 9 months. After years in storage, covid affecting business and the market being flooded with chinese replicas, this equipment now has a market value of R25 000. Since he's demanding R18,000, he is basically telling me he is paying R7000 for the usage period of 9 months is fair. He refuses to take the equipment back and pay the balance as per the sales agreement. I want your advice on what is the Shariah way to handle this matter?