75 ohm cable into a 50 ohm transmitter gives an SWR between the transmitter and the cable of 1.5 to one, if I recall my introductory course correctly. This is really not very much and might work OK depending on the transmitter, power, and the match at the other end to the antenna. A 1.5 to one SWR gives a transmission loss of 0.18 db according to the chart at this site: VSWR
If your transmitter has a transmatch built in, as many commercial HF rigs do, the SWR can be bought down to 1:1 anyway.
Less than 0.2db due to impedance mismatch is surely inconsequential loss and for instance common RG8X has a loss at 100mhz of 3.6 db per 100 feet vs only 2 db per 100 for RG6. So the SWR loss may be paid for by less loss on the transmission line depending on the situation.
Common cables transmission loss table here: http://www.w4rp.com/ref/coax.html