ولا يصح السلم إلا مؤجلا ولا يجوز إلا بأجل معلوم
Translation: Salam is valid only with deferred delivery, and only for a known period.
Ajal ma'lum is a delivery period fixed so that the time when performance becomes due is identifiable. “When the harvest is ready” or “whenever convenient” may leave the crucial date uncertain. An agreed calendar date or precisely calculable period avoids that defect.
Quduri also requires salam itself to be deferred, distinguishing the classical salam contract from an ordinary immediate sale. An agreement for immediate delivery needs to be analyzed under its own appropriate contract rules.