Follow these steps to purchase and send Bitcoin directly from your Cash App balance.
Open Cash App. On the main payment screen, tap the Money / Home button in the bottom left corner of the navigation bar.
Scroll down to the Save & Invest section and tap Bitcoin.
On the Bitcoin screen, tap the Buy button at the bottom of the screen.
Input the dollar amount you want to purchase and tap Next.
Back on the Bitcoin screen, tap the ↓↑ arrows button. Paste the exact BTC amount from your invoice into the field and confirm the send.