Hello shiva,
Yes, your allotted seat in JoSAA seventh round will be retain to you after participating in CSAB special round. Your JoSAA registration fee will be safe and it will be include in your tuition fee in your allotted college later. If you want participate again in CSAB special round then you have to pay registration fee although you got a allotted seat in JOSAA 7th round. Hope it helps for you.
Thank you.