If you want to assign a single record value of field Date to a variable, you need to use inter record functions like Peek().
For example, to retrieve the first Date field value in your [Table A] table:
LET vLOADDATE = Peek('Date',0,'TABLE A');
or to get the last record value:
LET vLOADDATE = Peek('Date',-1,'TABLE A');
Depending on the Date format, you would then need to use
from TABLE B where DATEPRICE = '$(vLOADDATE)';