Responsa על בבא מציעא 152:5

A - (1) Insofar as A is required to pay the scribe only for work that had been completed by the latter, the former may abrogate the agreement, even if the scribe is unable to procure other employment. However if people, who possess similar means as A, commission scribes to undertake such projects, then A can no longer assert, that due to financial circumstances, he must now cancel the agreement.