diff --git a/internal/money/money.go b/internal/money/money.go
new file mode 100644
index 0000000..b25a248
--- /dev/null
+++ b/internal/money/money.go
@@ -0,0 +1,89 @@
+// Package money represents amounts as int64 minor units + a currency code.
+// CNY/USD = 分 (1e-2), USDT = micro (1e-6). No float, no "元 string" (v2).
+package money
+
+import (
+	"errors"
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+)
+
+var ErrUnknownCurrency = errors.New("money: unknown currency")
+
+var exponents = map[string]int{
+	"CNY":  2,
+	"USD":  2,
+	"USDT": 6,
+}
+
+// Exponent returns the number of decimal places for a currency.
+func Exponent(currency string) (int, bool) {
+	e, ok := exponents[strings.ToUpper(currency)]
+	return e, ok
+}
+
+// Format renders minor units as a decimal string, trailing zeros trimmed.
+func Format(minor int64, currency string) (string, error) {
+	exp, ok := Exponent(currency)
+	if !ok {
+		return "", ErrUnknownCurrency
+	}
+	if exp == 0 {
+		return strconv.FormatInt(minor, 10), nil
+	}
+	neg := minor < 0
+	if neg {
+		minor = -minor
+	}
+	div := int64(math.Pow10(exp))
+	whole := minor / div
+	frac := minor % div
+	s := fmt.Sprintf("%d.%0*d", whole, exp, frac)
+	s = strings.TrimRight(s, "0")
+	s = strings.TrimRight(s, ".")
+	if neg {
+		s = "-" + s
+	}
+	return s, nil
+}
+
+// Parse converts a decimal string to minor units for the currency. It rejects
+// values with more fractional digits than the currency allows.
+func Parse(s, currency string) (int64, error) {
+	exp, ok := Exponent(currency)
+	if !ok {
+		return 0, ErrUnknownCurrency
+	}
+	s = strings.TrimSpace(s)
+	neg := strings.HasPrefix(s, "-")
+	s = strings.TrimPrefix(s, "-")
+	intPart, fracPart := s, ""
+	if i := strings.IndexByte(s, '.'); i >= 0 {
+		intPart, fracPart = s[:i], s[i+1:]
+	}
+	if len(fracPart) > exp {
+		return 0, fmt.Errorf("money.Parse: %q exceeds %d dp for %s", s, exp, currency)
+	}
+	if intPart == "" && fracPart == "" {
+		return 0, fmt.Errorf("money.Parse: empty %q", s)
+	}
+	whole, err := strconv.ParseInt("0"+intPart, 10, 64)
+	if err != nil {
+		return 0, fmt.Errorf("money.Parse int %q: %w", s, err)
+	}
+	fracPart += strings.Repeat("0", exp-len(fracPart))
+	var frac int64
+	if fracPart != "" {
+		frac, err = strconv.ParseInt(fracPart, 10, 64)
+		if err != nil {
+			return 0, fmt.Errorf("money.Parse frac %q: %w", s, err)
+		}
+	}
+	minor := whole*int64(math.Pow10(exp)) + frac
+	if neg {
+		minor = -minor
+	}
+	return minor, nil
+}
diff --git a/internal/money/money_test.go b/internal/money/money_test.go
new file mode 100644
index 0000000..d99044f
--- /dev/null
+++ b/internal/money/money_test.go
@@ -0,0 +1,47 @@
+package money_test
+
+import (
+	"testing"
+
+	"github.com/wangjia/pay/internal/money"
+)
+
+func TestParseFormatRoundTrip(t *testing.T) {
+	cases := []struct {
+		s, cur string
+		minor  int64
+	}{
+		{"12.3417", "USDT", 12341700},
+		{"0.01", "CNY", 1},
+		{"29.99", "USD", 2999},
+		{"199.99", "CNY", 19999},
+		{"1", "USDT", 1000000},
+	}
+	for _, c := range cases {
+		got, err := money.Parse(c.s, c.cur)
+		if err != nil || got != c.minor {
+			t.Fatalf("Parse(%q,%s)=%d,%v want %d", c.s, c.cur, got, err, c.minor)
+		}
+		back, err := money.Format(c.minor, c.cur)
+		if err != nil {
+			t.Fatalf("Format(%d,%s): %v", c.minor, c.cur, err)
+		}
+		// round-trip 回同一 minor(去尾零后可能字符串不同,再 Parse 校验)
+		again, _ := money.Parse(back, c.cur)
+		if again != c.minor {
+			t.Fatalf("round-trip %d → %q → %d", c.minor, back, again)
+		}
+	}
+}
+
+func TestParseErrors(t *testing.T) {
+	if _, err := money.Parse("1.00", "JPYX"); err == nil {
+		t.Fatal("未知币种应报错")
+	}
+	if _, err := money.Parse("0.001", "CNY"); err == nil {
+		t.Fatal("超精度(CNY 3 位小数)应报错")
+	}
+	if _, err := money.Parse("abc", "CNY"); err == nil {
+		t.Fatal("非数字应报错")
+	}
+}