Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: To make a sale saying, "I will pay when the stipend comes, or when the crop is harvested, or when the threshing floor is set up, or when the grapes are pressed," such a sale is not permissible; rather, a term should be fixed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11115