What do you want out of this?
What will happen if you fail the trial? Will you be offered another job or will you be made redundant? Will the job be readvertised as full time?
Can the job be done in 3 days? Would you do it full time?
Are there other ways round it like additional training for junior staff, formal procedures to follow and contact you in emergencies?
Where has the additional budget gone?
Can you sit down with your manager and discuss with him rather than doing it by email?
If posts have been deleted, has there been systematic changes in the service that have not actually resulted in reduced work load, can this be referred back to the people who carried out the restructuring work load assessment or are there changes that need temporary additional support to be implemented?
Can you suck it up for a month and then just work your 3 days and find ways of shoehorning it in? If you want to be paid for your extra hours, can you renegotiate for 4 days, would that work?