A300 LNDG BOI HAS CONFLICT WITH PRECEDING B757.

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WE WERE BEING VECTORED TO THE E RWY FOR A LNDG AT BOI. THE FO WAS FLYING TO AN ASSIGNED HEADING OF 280 DEGS (DOWNWIND) AT 8000 FT MSL. THE ACFT WE WERE FOLLOWING WAS A B757. JUST AS WE ROLLED OUT OF THE 280 DEG HDG THE APCH CTLR GAVE US A CLRNC TO DSND TO 5000 FT MSL. THE WX CONDITIONS WERE IMC (IN AND OUT OF CLOUDS) WITH VISIBILITIES 2-3 MI. AS WE WERE DSNDING THE B757 WAS GIVEN A 270 DEGS L TURN TO SET THEM UP FOR A LNDG TO THE W. THIS WAS DUE TO A SUDDEN CHANGE IN THE SURFACE WINDS. AS THIS ACFT COMPLETED ITS TURN THE CTLR TOLD US TO LEVEL AT 7000 FT MSL. BY THIS TIME WE WERE ALREADY AT 5600 FT MSL DSNDING TO 5000 FT MSL. AT THIS TIME THE TCASII DISPLAYED AN RA SIT AND COMMANDED AN IMMEDIATE CLB OF 2000 FT OR BETTER. THIS WAS ACCOMPLISHED AND THE B757 STATED THAT HE WAS DSNDING. AFTER LNDG I CALLED RAPCON AND TALKED TO THE CTLR. HE STATED THAT 'HE INTENDED TO HAVE OUR ACFT DSND TO 7000 FT MSL VERSUS 5000 FT MSL AND HAVE THE B757 CROSS UNDER US ON HIS BASE LEG.' THE TAPES WERE PULLED AND CONFIRMED THAT WE WERE GIVEN A CLRNC TO 5000 FT VERSUS 7000 FT MSL AS HE HAD INTENDED.