Final answer:
Olsen remains liable to Able for the $10,000 despite paying Archer, because after the assignment was communicated to him, his obligation to pay shifted to Able.
Step-by-step explanation:
When Olsen's contract with Archer was validly assigned to Able, and Able notified Olsen about the assignment, Olsen's obligation to pay the $10,000 shifted to Able. Olsen's payment to Archer, after the assignment had been communicated to him, does not discharge his obligation to Able. Therefore, even after making an erroneous payment to Archer, Olsen remains liable to Able for the $10,000. This is the general rule unless the parties agree otherwise or state statute dictates a different outcome. Additionally, the issue of bankruptcy does not negate Olsen's liability to Able. It is also important to note that notification of assignment does not necessarily need to be in writing unless specified by statute or the original contract.