If you have $25k cash in your account and the margin offered your broker is 50% then you have $50k of buying power (not $75k). At 50%, your broker will lend you an amount equal to the total of your cash and marginable securities. Here's the long margin calculation:
Market Debit Equity Margin
50,000 25,000 25,000 50.00%
I short sale stock worth $5000. How much of initial cash is now available for buying securities after shorting?
The Reg T short margin requirement is 50% of the value of the short sale ($2.5k). That leaves $47.5k for margin buying.
The shorted stock is now worth $10,000. How much of initial cash is now available for buying securities? What would be my buying power now? Would would be my margin amount available now?
Reg T margin maintenance requirement for a short position is 30%. At $10k, that's $3k maintenance. Your short position lost $5k so account value dropped to $20k ($40k BP) and with the MMR of $3k, you'd have $37k available for long margin buying.
Another way of looking at this is that you started with $50k of buying power. The initial margin for your short is $2.5k. When the short doubles to $10k, you have to add another $5.5k to meet MMR. That's $8k. With the $5k loss of equity, that's $13k. $50k - $13k equals $37k of buying power.
Short Sale Initial Total
Value Margin Marg Req
5,000 2,500 7,500
Short Sale 30% Maint Total Margin
Value Margin Marg Req Call
10,000 3,000 13,000 5,500
Note that brokers can require more margin than Reg T.