Under Florida law, payday lenders cannot seek to have a borrower prosecuted for bouncing a check given as security for a loan, so if the creditor is threatening to have you charged with "theft by check" it is almost certainly lying in an attempt to frighten you.
Although payday loans are legal in Florida, the amount of interest and other fees they can charge to consumers are fairly strictly regulated by state law, so I find it highly unlikely that this lender will be able to justify its claim that you owe $6,000 on two loans which began at only $300 each. To read more about payday loans and your rights as a consumer, I encourage you to visit PaydayLoanInfo.org. There, you can also review a summary of Florida's Payday Loan Regulations.
Under Florida law, the payday lender may be required to work out a repayment plan with you at a reasonable interest rate (my understanding is that it can charge no more than 10% per annum on loans after the first high interest repayment period). If the lender continues to insist on the high balance which you mention in your question, I recommend that you demand a written statement of how the balance was calculated before you make any payments to the creditor. If the creditor refused to provide you with a written explanation of the balance claimed, or if it continues to threaten you with criminal prosecution if you fail to pay the amount claimed, I encourage you to file a complaint with the state agency responsible for the regulation of payday lenders, at The Florida Office of Financial Regulation.
From the information in your question, it sounds like this lender may be violating several state laws regulating payday lenders. However, without much more information I cannot tell you specifically what recourse you have available to you. If you find that the lender is unwilling to work with you, or if you do not feel comfortable negotiating with the creditor directly, you may want to consult with an attorney who can better explain your rights under Florida law and assist you in negotiating with the lender.
The National Association of Consumer Advocates (NACA) may be able to refer you to an attorney in your area who specializes in assisting consumers with problems like yours. See also my answer to another reader who had a question about aggressive payday loan collections in Florida in "Payday Loan and the FDCPA."
Bills.com also offers more information on the Payday Loan Information page, and has answered reader questions about payday loans in California, Florida, Illinois, Massachusetts, Missouri, New York, Texas, and Virginia.
If you do not repay a payday loan, the payday loan company has several legal remedies, including wage garnishment, levy, and lien. See the Bills.com resource Collections Advice to learn more about the rights of creditors and debtors.
I wish you the best of luck in resolving these payday loans, and hope that the information I have provided helps you Find. Learn. Save.