Short answer. No. A claim that arises out of, or is connected with, the same transaction the plaintiff is suing you over is a compulsory counterclaim, and the Rules of Court bar it if you do not raise it in the same action. Saving it for a separate case later usually means losing it entirely.
What the law says
A compulsory counterclaim not raised in the same action is barred, unless otherwise allowed by these Rules.
Rule 6, Section 7 — Compulsory counterclaim. Read the full provision →
What the law says
A compulsory counterclaim, or a cross-claim, not set up shall be barred.
Rule 9, Section 2 — Compulsory counterclaim, or cross. Read the full provision →
What makes a counterclaim compulsory
Rule 6, Section 7 defines a compulsory counterclaim as one that arises out of or is connected with the transaction or occurrence constituting the subject matter of the opposing party's claim. Two further conditions attach: it must not require for its adjudication the presence of third parties of whom the court cannot acquire jurisdiction, and it must be within the court's jurisdiction as to both amount and nature — although in an original action before the Regional Trial Court, the counterclaim may be considered compulsory regardless of the amount. If your claim against the plaintiff grows out of the very deal or incident you are being sued over, it almost certainly falls within this definition.
The bar is written into the Rules twice
The consequence of silence is spelled out in the same section: A compulsory counterclaim not raised in the same action is barred, unless otherwise allowed by these Rules. Rule 9, Section 2 repeats it — a compulsory counterclaim not set up shall be barred. Barred means exactly what it sounds like: once the first case runs its course, you cannot bring the saved claim as a fresh suit. The bar does not depend on whether you win or lose the main action, and it does not matter that your claim might have been strong. The Rules treat the first case as your one opportunity to litigate everything arising from that transaction.
Unrelated claims can wait
The bar reaches only compulsory counterclaims. A claim against the plaintiff that arises from a different transaction — an unrelated loan, a separate contract, an earlier incident — is not caught by the definition, and you may sue on it separately without losing it. The dividing line is the connection between your claim and the subject matter of the plaintiff's suit, so the practical question is whether both claims rest on the same facts. When they do, treat yours as compulsory: guessing wrong in the other direction costs you little, while guessing wrong here forfeits the claim.
Raise it in your answer
If the claim is compulsory, plead it in the same case — the safe course is to include it in your answer rather than gamble on its classification. Gather the papers that prove it: the contract, receipts, correspondence, and a record of the losses you suffered. The Rules do leave a narrow opening, since the bar applies unless otherwise allowed by these Rules, but that is nothing to build a strategy on. A lawyer will need to see how your claim connects to the plaintiff's before advising whether it is compulsory or permissive, and that assessment belongs before your responsive pleading is filed, not after the case ends.