No, you cannot enforce an oral promise (that's the better term than "verbal") for a bonus. The problem is, all employment is employment at will which means, among other things, an employer can change compensation (e.g. promise a raise or bonus, then renege on that promise) at will, *except* and only to the extent that there is a written employment agreement setting or guarantying the compensation. When there is nothing in writing, the employer may change its mind--and that is especially so when there was a written letter which did not mention the bonus. In that case, a court would conclude that the failure to include the bonus in a writing which was otherwise being issued (the offer letter) means that the promise did not exist, since when there is a written agreement of some kind, it supercedes all prior oral promises, and promises which did not make it into writing are not enforceable.