Due to Sustainability Consult (70272213) has never been used in 6020,so it must be created first then you can process by SRM7.
According to your question concerning partial payment of invoice that vendor will issues only one invoice and we will make a partial payment, you have to create transaction in SRM7 with full amount and authorized person also approve with that amount in SRM7 and further create Purchase Order.
For partial payment,you have to control the payment by yourself,you have to do monthly GR together with sending PRQ to SBS PtP for payment
Furthermore,we have a contract with vendor or not ? if there is please share with us to check the tax issue that may be associated with consideration of that contract.