Allow negative price for products

api/db/product.go

@@ -55,9 +55,7 @@ func (d *DB) UpdateProduct(code int, product *Product) (dbProduct Product, err e
 	if product.Name != "" {
 		dbProduct.Name = product.Name
 	}
-	if product.Price >= 0 {
-		dbProduct.Price = product.Price
-	}
+	dbProduct.Price = product.Price
 	if product.Stock >= 0 {
 		dbProduct.Stock = product.Stock
 	}