Is it permissible to fulfill a vow with prayer performed intermittently after the fortieth day, knowing that the vow was for 100 rak'ahs to be prayed all at once immediately after the fortieth day?

Undertaking a vow is disliked (makrooh). A vow is contractually binding when expressed in a formula indicating commitment, such as "It is Allah's due upon me thus and thus," or "I have vowed thus and thus." If the vow is coupled with "in sha Allah" (if Allah wills), then fulfilling it is not obligatory according to most scholars. However, if the phrase "Allah wills" is not uttered, or if it is not connected to the wording of the vow, then fulfilling the vow at its appointed time becomes obligatory. Delaying a vow beyond its time does not necessitate expiation according to the majority of scholars, but it remains a liability that must be fulfilled in the manner vowed, and partial fulfillment is not sufficient. If the one who made the vow is unable to fulfill it due to an inability that is not expected to cease, then he must offer an expiation for breaking an oath. As for a mere promise that was not expressed in the formula of a vow, fulfilling it is not obligatory, but only recommended.