NEW YORK, July 13 (Reuters) - AG Mortgage Investment Trust Inc, a real estate investment trust (REIT) managed by Angelo Gordon & Co, made a stock-and-cash offer on Thursday for Western Asset Mortgage Capital Corp, a peer managed by Franklin Resources Inc, that values it at about $300 million, including debt.

If successful, AG Mortgage's bid would disrupt an all-stock deal that Western Asset Mortgage unveiled on June 28 to merge with Terra Property Trust Inc (TPT), a REIT managed by Mavik Capital Management LP.

The deal with TPT valued Western Asset Mortgage at the time at $17.30 per share. Western Asset Mortgage shares have traded significantly lower since, closing on Wednesday at $8.35. This is because investors have been skeptical about the value of TPT's shares, which do not trade on any major stock exchange.

AG Mortgage's offer is equivalent to $9.88 per share, an 18.2% premium, according to a letter AG Mortgage sent to Western Asset Mortgage on Thursday that was reviewed by Reuters. The consideration consists of $8.90 per share payable in AG Mortgage shares and $0.98 per share payable in cash.

"(Our) stock trades on the NYSE with an observable value, while TPT's shares are not listed and have never been valued by public markets through a stock exchange listing," AG Mortgage CEO T.J. Durkin wrote in the letter.

Western Asset Mortgage did not immediately reply to a request for comment on the offer.

AG Mortgage also offered to waive about $2.4 million of management fees after the deal closes, and proposed to expand its board to include two of Western Asset Mortgage's independent directors. AG Mortgage added that its proposed deal is not subject to any financing condition.

Western Asset Mortgage invests in residential whole loans and mortgage-backed securities, as does AG Mortgage. Angelo Gordon, a credit and real estate-focused investment firm which currently manages about $73 billion in assets, agreed in May to sell itself to private equity firm TPG Inc for $2.7 billion. (Reporting by Anirban Sen in New York; Editing by Stephen Coates)