Van't Schip admits the young defender had also attracted interest from Eredivisie clubs before being sold to City for a record €3m.
The Dutchman had worked for City Group as coach of their Australian partners Melbourne City and tipped them off about Sandler earlier this season.
He revealed: "It is true that I spoke with Txiki Begiristain.
"At the moment that AZ and PSV were interested, they watched him too, so we have said: 'Look at him and discuss it for a while, he could also be an interesting player for you'."
Van't Schip said of Sandler: "He has made enormous strides in defence, physical strength, a good header, a boy who could play football for Manchester City in the long run."