An airline cannot "FORCE" you to give up anything at another company. They have NO business interfering with your relationship with your other company and cannot force the other company to DENY you your seniority rights.

Besides, the company you are leaving can choose to ignore any "resignation" letter that a new outfit tells you to send. They are not bound by any obligation to your new employer.

Your best bet is to talk to your present employer and get confirmation (preferably in writing) about what your rights are if and when you want to come back. This agreement is NONE of your new employer's business and they are not entitled to interfere with such an agreement between you and your old boss. DON'T let them tell you otherwise.