Do I have the right to the same pay I was previously on if I ask to end my secondment early?

I took up a secondment role at another school in our multi-academy trust.  This commenced from September 18 and was to be for a year.  I have asked for the secondment to end after one month due to stress related issues at the host school which has been agreed and I have now been returned to my original school.  I previously was paid on upper payscale 3 and had a teaching and learning responsibility which carried an extra payment.  This has been my pay for the last 2 years.  I have a letter stating that my job with the teaching and learning responsibility would be'kept open' for me as I took up the secondment.  I am now being told that I have no entitlement to the TLR as the school was not expecting to have to pay me.  I signed no paperwork relating to the secondment and was not told I would lose pay when I asked to end the secondment early.