The regulator received a complaint from the journalist who wrote an article in The Guardian newspaper. It said that the directors of the company that runs the scheme have earned almost £10m from it since 2008.
It claimed that Nigel Hanger, owner of Kettering Textiles, which runs the charity’s clothing banks and door-to-door collection scheme, had earned £5m from collections over the past five years.
A statement from the FRSB said charities should be open and transparent "in all their fundraising efforts and across all public touch points".
It said this included solicitation statements on clothing collection banks and bags.
"Charities need to do all that they can to educate the public in how fundraising works," it says. "This includes the use of consultants or third parties in helping them raise much-needed funds.
"Openness and transparency are vital for engendering public trust and confidence in charitable giving."
The FRSB does not normally publicise complaints so early. But Alistair McLean, its chief executive, told Third Sector that after the article was published on Monday it had received a large number of enquiries and had decided it was best to update people on what was happening.
McLean said he hoped the charity would be able to resolve the complaint, but that if it could not, the FRSB’s board would step in to investigate, as it usually would.
The Salvation Army declined to comment on the complaint, but Lieutenant Colonel David Hinton, chair of the Salvation Army Trading Company board of directors, said in a statement that all of the facts reported in The Guardian's article were in the public domain.
"The commercial relationship between SATCoL and Kettering Textiles is outlined on the SATCoL website and on all door to-door collection bags," he said.
"A full statement regarding the relationship is currently being rolled out on the SATCoL recycling banks, which the FRSB have confirmed is fully compliant with current fundraising legislation."
A spokeswoman for the Charity Commission said it was considering issues that had been raised with it to see if it had a regulatory role. The regulator had been in touch with the FRSB, she said.