Presumably, the agreement (whether written or oral) made before or after you let your employer use your card was that he would reimburse you for all  expenses; if so, and you had not agreed in advance to simply gift the purchases to your former employer, you may sue him for breach of contract. That is, you may bring a lawsuit to recover all the money from him, pursuant to the agreement that you would lend him your card and he would repay all purchases; for amounts less than $2,500 or so, you're better off suing in small claims and acting as your own attorney; for more than that, you may wish to retain a lawyer.