Can you play Cruel Patron to a battlefield if you sacrifice your only unit (a recruit) at that battlefield as the cost?

Ruling: No, you cannot play Cruel Patron to a battlefield if sacrificing your only unit there as a cost would cause you to lose control of that battlefield, because this would deterministically result in illegal choices later.

Sequence:

• You declare intent to play Cruel Patron and choose the battlefield location
• You pay costs (killing your only unit at that battlefield)
• A cleanup occurs (either immediately after paying costs or between finalization and resolution)
• You lose control of the battlefield because you have no units there and it's not contested
• The chosen location is now illegal, making the play invalid

Nuances:

• Control of a battlefield is a passive state that changes immediately when you have no units at an uncontested battlefield
• The cleanup happens in the middle of playing the card (after finalization but before resolution, or potentially during cost payment)
• Even though the choice was legal when made in step 2, it must still be legal after costs are paid
• "Valid Location" is not currently well-defined in the 1.1 core rules, which is a known oversight