"Promissory" qualifies "agreement", not "quota": in other words, this expression does not refer to a "promissory quota", but to a "promissory agreement", which is a kind of contract in which one party promises to do something in the future in return for consideration.

"Quota assignment" tells us what sort of agreement this is: it is an agreement to assign a quota. A quota means a share of something. In principle it could be, for example, a production quota or an import quota, but it probably refers to shares (stock) in a company. To "assign" means to transfer ownership. So this is a quota assignment agreement (QAA), an agreement to transfer ownership of (probably) company shares. But it is a promissory QAA, not a definitive QAA, so this agreement does not actually transfer ownership of the quota or shares; it embodies a promise or undertaking by one party to assign a quota to the other party at some point in the future.