Am I responsible for a stock option or bonus plan promise the seller made to an employee?

In a share purchase, generally yes — a promise the corporation made to an employee, whether documented in a formal plan or a less formal arrangement, is a commitment of that corporation, and since you now own it, honouring or resolving that promise is your responsibility going forward, subject to whatever the actual plan terms or agreement say and any protections you negotiated in your purchase agreement.

In an asset purchase, this generally depends on whether you specifically assume the obligation. A promise the seller made personally, or under a plan tied specifically to the selling corporation, doesn't automatically transfer to you as a separate legal entity unless your purchase agreement says it does, or unless you choose to honour it as a matter of retaining a valued employee, which is a business decision separate from any legal requirement to do so.

Because bonus and equity promises are sometimes informal or inconsistently documented, especially in smaller businesses, review employee compensation arrangements carefully during due diligence rather than relying only on formal written plans, and decide deliberately what you're taking on. A Treadstone business lawyer can help review these arrangements and clarify what transfers.