QUESTION:
Our employer has issued payslips for 40% of our salary (part of an agreement, see below) but it telling us to ignore those, that they overpaid us last month and recovering that from this month's payroll, reducing us to an effective 17% of our salary with only informal (whatsapp) messages and no formal communication. Do they have to honour the payslip for this month?

BACKSTORY:
After a difficult year and some very accommodating employers (give the circumstances) my employer negotiated with us as staff an agreement to pay a TERS related amount, pending the successful outcome of a TERS application, and if the application wasn't successful we agreed to a collective payment reduction of 40%.
The employer applied immediately in August and expected a 30 day turnaround. THe fund asked for additional documentation which they only supplied mid-November, at the same time we were sent a formal letter after the beginning of November that despite continuing to pay the amount agreed expecting a successful TERS application for September and October, from November they were going to put us on 40%. We did not fight it, however a few days later our payslips arrived at the full TERS expected amount (not the 40%). No follow up communication at all despite pointing out to my immediate superior what had happened. Yesterday we got a whatsapp message from a superior indicating that we were overpaid in November, we should have only received 40%, so they would deduct that overpay from Decembers payroll. No formal notification at all. This morning my payslip arrived at 40%. When raised with the HR administrator they said we should ignore our payslips. So, my question is, once a payslip is issued, does the company have to honour it? I know they can deduct overcalculations, but surely there is some level of formal communication and engagement required around this? Any advice/answers?

Thanks