Selectmen have asked for more information on a Water and Sewer Commission  plan to borrow the $7.5 million  needed to become a partner with Mansfield and Norton in the proposed expansion of Mansfield’s wastewater treatment plant.  The request broached by commission chairman Bill Euerle  at the most recent selectmen’s meeting proposes is to invest in the operation, rather than continue on as a customer. Under the proposal, if eventually endorsed by selectmen, residents would be asked at a special fall Town Meeting for permission to borrow the money. Unlike twice in the past, they would not be asked to pay betterments for the new system. Instead, officials said, the town’s capacity would be increased by 240,000  gallons per day. Selectmen and others have long said that sewer expansion is the key to economic development, particularly in the town center, where capacity has long been reached.