As your brother purchased all of the share capital personally then your brother is liable for the remaining instalment. If after purchase your brother dumped the company in the middle of the ocean, changed its name and country of registration, immediately sold it to someone else, stopped trading straight away or sent it to the moon it would alter nothing, your brother is liable for full payment. The fact that the vendor invoiced the instalments to the company name and was paid in the company name alters nothing, it is only a matter of convenience and does not void the basic contract.