I suggest that you consult with an attorney who practices in the area of contract law to go over the documentation that you have concerning your contractual obligations to the United States Navy with respect to the signing bonus you received and whether or not you are obligated to pay it back or not. Only from looking at the written documentation and any terms for re-payment of the bonus by the suggested attorney will you be able to see if there is some way you do not have to re-pay the "bonus" that you received but is demanded to be returned.