Trey Songz says a Texas concert promoter left him hanging before a scheduled show ... and now he's suing for more than half a million bucks.

According to a new lawsuit, obtained by TMZ, Trey claims his production company struck a deal with promoter Darryl Austin in April for Trey to perform at Will Rogers Auditorium in Fort Worth on August 8.

Trey's company says Austin made the first required payment ... but allegedly failed to make the second installment due July 8, then asked to postpone the concert.

The suit claims Trey's team believed the promoter hadn't done enough to market the show -- despite repeated offers to help -- and refused the postponement request, demanding payment instead.

According to the complaint, Trey was still ready and willing to perform ... but Austin allegedly postponed the concert anyway ... with Ticketmaster telling fans the event had been pushed back without explaining who made the call.

Trey's company says the alleged breach cost it more than just the unpaid fee ... claiming lost meet-and-greet revenue, damage to its professional goodwill and harm to future earning power.

It's seeking more than $500,000 in damages ... plus legal costs and anything else the court sees fit.