You are correct @mf_bib – as mentioned in the documentation, the float32 writing is not working with the openhab1 addon
Note: The way Decimal commands are handled currently means that it is probably not useful to try to use Decimal commands with non-16bit valuetypes.
If you feel lucky, you can try out the experimental/new openhab2 modbus binding. The new version should handle float32 writes correctly.