VFP relations between cursrs do not work well with buffering mode. This because they use physical indexes, that will not be updated for buffered tables until you use tableupdate() function. I don;t know however, why you receive locking errors, but I suggest because buffered record was not updated automatically when you moved to next record in alias.
To remove any buffering, just set buffering mode to 1. Use "cursorsetprop('Buffering',1,'alias')" function for this. Should work fine, AFAIK from experiense with FoxPro 2.6 where I used such relations very often.