The dictionaries do not attempt to include all verb forms of a verb, so I wouldn't expect to see all those forms anyway. However, as you have already discovered, only the noun phrase (back pay) is accepted currently. Spoken language normally precedes the written language (internet coinages nowadays excepted), so although you have heard the verb, it is still considered casual English, for which a supervisor would (or should) still write 'The company will be gving him back pay'.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.