Tds payment on booking (purchase order) -provisonal income

Hello Team

We have observed that many customers pay TDS based on Purchase order ie Booking amount and pay that TDS in year ending March. when we see the 26AS we understand that TDS is paid on Booked amount which is not billed ie Invoiced and hence doesnt reflect in our Sales ledger.Note this provision is expense which will be incurred in fortcoming year

My query is as follows

1. Is the parctice followed by Customer Ethical

2. In any case can we take benefit of the TDS deduction ( we have not shown income)

3.In the fortcoming year when billing is done and payment is made to us theres no deduction done . Excess Deductions are done on payments done in earlier year.

Thanks!