Salam is a contract in which the buyer pays the full price now for a well-defined commodity to be delivered at a specified future date, and it is permissible by explicit text as an exception to the general rule against selling what one does not yet possess. Its validity depends on the goods being described precisely by measure, weight, and quality, and the delivery term being fixed. When the Prophet (ﷺ) came to Madinah he found people paying in advance for fruits, and he instructed: "Whoever pays in advance for something, let him do so for a known measure, a known weight, and a known term." It is widely used in Islamic finance today to fund farmers and manufacturers without falling into riba.
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.